Electric signal transmission line



' July 4, 1939. A. D. BLUMLEIN ET AL 2,164,492

7 ELECTRIC SIGNAL TRANSMISSION LINE Filed March 4, 1936 2 Sheets-Sheet 1V i l E L R R? ksi lfia/F f' 'fl L553 i f F642 July 4, 1939. A. D.BLUMLEIN ET AL ELECTRIC SIGNAL TRANSMISSION LINE 2 Sheets-Sheet 2 FiledMarch 4, 1936 FREQUENCY If,

FREQUENC Y M02 TLMBUQ 0 Inventors ALcm Dower BLumLeiu & John HarlwieliPatented July 4, 1939 PATENT OFFICE ELECTRIC SIGNAL TRANSMISSION LINEAlan Dower Blumlein, Ealing, London, and John,

Hardwick, West Drayton, England, assignorsto Electrical & MusicalIndustries Limited, Hayes Middlesex England, a company of Great BritainApplication March 4,

1936, Serial No. 67,048

In Great Britain March '7, 1935 8 Claims.

The present invention relates to electric signal transmission lines andmore particularly to transmission lines which are required to handlesignals covering a wide band of frequencies,

In a transmission line havingdistributed resistance, inductance andcapacity the propagation constant P is given by where R, L and C are theresistance, inductance and capacity per unit length of the linerespectively and represents the angular frequency of nal beingtransmitted and is equal to 21rf is the frequency in cycles-per second.

Yv'hen w has a high value so that wL is much greater than R thepropagation constant P is given by P=jw approximately In this equationline is given by (.0 W B in this case we have The wave velocity W istherefore independent of frequency. Further, the second term in theexpression for Pin Equation 1 denotes the attenuation constant which isalso independent of. frequency. The line is therefore distortionless,when w is large.

Equation 1 however, can only be applied when w has a high value. If w issmall it is found that both the wave velocity and the attenuationconstant are functions of w and the line is, therefore, notdistortionless. The variation of wave velocity and attenuation constantwith no at low frequencies will be termed the low frequency effect. Evenwhen ml; is large compared with R, there is another efiect which mayintroduce distortion. This is the change of the so-called cableconstants L and R at high frequencies due to the skin effect. A changeof these constants with frequency introduces changes in the propagationconstant with frequency and distortion results. Thiswill be called thehigh frequency effect.

It is an object of the present invention to provide a new or improvedelectric transmission system for transmitting electric currentsextending over a wide band of frequencies, said system comprising atransmission line wherein, distributed along the length of the line,there are provided impedance elements of such nature and magnitude andso connected and arranged as to increase the attenuation of saidtransmission line over a substantial part of said band of frequenciesand to render the attenuation and Wave velocity of said transmissionline substantially constant over said band of frequencies.

The invention will now be described by Way of example with reference tothe accompanying diagrammatic drawings in which Fig. 1 shows atransmission systemaccording to the invention,

Figs. '2 to 5 show details of various arrangements according to theinvention for equalizing a cable,

Fig. 6 shows an alternative arrangement according to the invention ofthe cable termination shown in Fig. 1, and

Figs- 7 and8 are explanatory figures showing graphs which will bereferred to in the course of the following description.

Reference is first directedto Fig. 2 of the drawings, in which atransmission line is shownas having two conductors l and 2. The seriesresistance of the line is denoted by R and the series inductance by L.For convenience, series impedances in the line are shown in oneconductor only. In practice the series impedances are distributedbetween the two conductors but if R and L denote the total resistanceand inductance per unit length the distribution between the conductorsdoes not affect the calculations. The shunt capacity of the line isdenoted by C and the shunt leakance by G. It will first be assumed thatG is so small that it can be neglected.

The components L, R, G, C, representing the cable constants are enclosedin a dotted rectangle 3 in order to distinguish them from added loadingimpedances.

The low frequency effect may then be subkilometre.

can be introduced in series with the central con- R 2 4 m) Thepropagation constant P is now given by The wave velocity and attenuationare then constant for all frequencies even when w is small as well aslarge provided L, C, C and R are independent of frequency.

The characteristic impedance of the line then becomes which is anexpression for the impedance of a condenser and resistance in series.

In practice it is usually necessary to arrange the series condensers atintervals, which intervals are preferably short compared to thewavelength, at

the frequency forwhich wL=R. For example a required loading of 1microfarad-kiometre may be made up by introducing condensers every halfFor a concentric line the condensers ductor, the above loading requiring2 microfarads every half kilometre. For a balanced line the condensersare preferably put in each conductor,

. e. g., 4 microfarads in each conductor everyhalf kilometre. For lineswithout added inductance loading, spacings between the condensersgreater than a kilometre may be satisfactory.

If G is not so small as to be negligibfe, then it is necessary to shuntthe capacity C by a leakance G.

It can be shown that the ideal values of C and G are as follows It maythen be shown that the propagation constant is given by and thecharacteristic impedance Z of the line is .an inductance shunted by aresistance.

This represents the impedance of a resistance in series with acondenser, the condenser being shunted by a resistance. 15

is greater than then is less than then This represents the impedance ofa resistance shunted by a resistance and inductance in series.

The high frequency effect which has been defined as the change of wavevelocity and attenuation of the line due to the change of inductance andresistance at high frequencies may be treated as follows.

The values L, C, R which have been given to the inductance, capacity andresistance of the cable are the values obtained by measurement at acomparatively low frequency, say 1000 cycles per second. At higherfrequencies, say one megacycle per second, the resistance of the line isgreater than R and the inductance is less than L. The cable constantscan be represented to any desired degree of accuracy by including inseries with the line a plurality of circuits each comprising In Fig. 3of the drawings three such circuits L1, R1, L2, R2 and L3, R3 are shown.

At very low frequencies the inductances L1, L2, L3 act as short-circuitsacross the resistances R1, R2, R3 and the effective resistance of theline is therefore R, the low frequency inductance being L+L1+L2+Lz. Itwill therefore be seen that the value of L is in this case differentfrom the value employed in the consideration of Fig. 2.

At higher frequencies the resistances, R1, R2, R3 act as short-circuitsacross the inductances L1, L2, L3 and the inductance therefore falls toa value L whilst the resistance rises to R+R1+R2+Ra. The attenuationtherefore increases and the wave velocity falls with increasingfrequency. In general, the primary constants of -a line can berepresented to any desired degree of accuracy by considering a pluralityof circuits such as L1, R1, L2, R2, etc. The number of circuits whichhas to be considered depends on the frequency range over which the cableis to be corrected and on the degree of approximation which is required.In general, the greater the frequency range or the closer theapproximation, the greater must be the number of circuits.

The variation of resistance and inductance at high frequencies is thencorrected by connecting condensers C1, C2, C3 in series with the line,the

v at frequency in.

condensers being shunted by resistances R1, R2, R3 respectively. Thevalues of the condensers C1, C2, C3 are given by equations C1L1=R1 etc.A capacity-resistance circuit is included to correspond to each of theinductance-resistance circuits into which the cable constants have beenanalysed.

The inclusion of the condenser-resistance circuits renders theresistance of the line invariant with frequency and equal to R+R1+R2+R3and the inductance term is also invariant with frequency and equal to L.This applies over the frequency range within which the circuits enclosedin the dotted rectangle 3 accurately represent the effective resistanceand reactance of the line. The elfect of this capacity-resistance lodingis to produce a line with substantially constant inductance, resistanceand capacity at all frequencies within the range over which the circuitsconsidered represent the constants of the cable.

Since the attenuationof a line equalised in the manner described abovemay be considerable, it is necessary to provide amplifiers at intervalsalong the length of a long line. In practice it is sometimes founddesirable to introduce attenuation and velocity equalisation upto afrequency which is somewhat lower than the highest frequency which theline is required to handle. Correction is then provided in theamplifiers for the increasing attenuation and falling velocity betweenthe frequency up to which the line has been corrected and the highestfrequency to be handled.

For example, if the cable constants can be represented to a-desireddegree of accuracy up to a frequency ,f by considering only tworesistance-inductance circuitsRi, L1 and R2, L2 and if it would benecessary to consider a third circuit R3, L3 in order to represent theconstants up to the highest frequency ft, to which it is desiredtooperate the line, then the line may be equalized up to frequency byincluding two capacity-resistance circuits 01 R1 and C2 R2. Theresistance of the line for frequencies up to is then R+R1+R2 and abovefrequency f the resistance rises from this value to R+R1+R2+R3 Similarlythe inductance has a value L+L3 at frequencies up to f and a value L atfrequency in.

By not equalising to the top frequencies, the noise in the middlefrequency range (due to Johnson noise and valve noise in amplifiers) isreduced and also loading at very short distances is avoided. On theother hand,.the complexity of the amplifiers is increased. It may befound in practice that it is more economical to load the cable tosubstantially constant-attenuation and velocity over the entire rangeand to reduce the repeater. distances slightly to allow for theincreased noise. It must be realised that the threeresistance-inductance circuits shown representing the variable cableconstants, are given by way of exampleand the cable covering a very widerange may require many more circuits to represent it to a particulardesired degree of accuracy.

Similarly, the type of capacity-resistance loading shown need notnecessarily be adhered to. When two different types of loading areinserted at one point, the equivalent electrical network to twocondensers in series each shunted by a resistance may be used. Forexample, one condenser shunted by a resistance, the whole being shuntedby resistance and condenser in series,

can by the proper choice of values, be made identically equal to two ofthe circuits shown in Fig. 3. There are many other possible equivalentcircuits which provide the same impedance with a physically differentarrangement of resistances and condensers. Such equivalent circuits arewell known in the art and an example of four such equivalent circuitsare shown on page 278 of Transmission Circuits for TelephonicCommunication by K. S. Johnson, second printing, published by the D. vanNostrand Company, 1925.

It may in many cases be found more convenient not to calculate thevalues of R1, L1, etc., at all, but to proceed directly to the determination of the values for R1, C1, etc. This may most conveniently be doneby measuring in well known manner the primary constants of the cableover the required frequency range.-

Referring to Fig. '7, curve 11 represents a series resistance of asmooth line, measured experimentally, and plotted with respect tofrequency, over the frequency range zero to in. Then in order toequalize the attenuation of the line over this frequency range, it isrequired to provide loading means the resistance of which varies withrespect to frequency as shown by curve I). The resistance of the loadedline over the frequency range zero to in will then be R0.

The required loading may. be experimentally determined asfollows: Byplotting to the same scale as that of curves a and b the variation withfrequency of the resistance of a circuit comprising a resistance andcondenser being determined experimentally, the curve can be obtained.Assuming that this circuit is. connected in series in the line; then theresistance-frequency characteristic of the loading which is stillrequired is shown by curve 11, which is obtained by subtracting curve 0from curve I).

From curve d, it is clear that the introduc tion of theresistance-capacity circuit whose frequency characteristic isrepresented by the curve 0 has had the effect of rendering theattenuation of the line substantially uniform up to the frequency f1,this frequency being lower than it.

Referring now to Fig. 8, a further resistancecapacity circuit isdesigned having a frequency characteristic shown by curve 6. Thiscircuit is also connected in series in the line, and the frequencycharacteristic, of the loading still required is given by curve g, whichis obtained by subtracting curve 6 from curve (I of Fig. 7. Theintroduction of this second resistance-capacity circuit results in anapproximate equalization of attenuation up to the frequency f2 lyingbetween the frequencies f1 and f0.

Similarly, a third resistance-capacity circuit is designed and connectedin series. in the line, this third circuit being designed to have afrequency characteristic such that the of the resistance of the linefrom the value R0 can be represented-by the curve h, whichwill be seento be co-inoident over a large part of the fre quency range zero to Jd'with the ordinate representing zero resistance.

If it be required to equalize the attenuation of the line up to afrequency greater than 3%, one

or more further series circuits, designed the manner set out above, areconnected in series. in the line.

The addition of condensers and resistances such as C1, R1, C2, R2 andC3, B3 increases the resistance of the cable at low frequencies, andincreases the magnitude of the low frequency effect. It thereforebecomesimportant, if satisfactory working at low frequencies isrequired,

Eli

to correct for this low frequency effect. In Fig. 2

the series condenser C which is used as already described for correctingfor the low frequency effect must have a value If the values used inthis equation are the values per kilometre, value of capacity shown isthat of the condenser which must be used if the loading is introducedonce every kilometre. For shorter distances of loading, a largercondenser is necessary. The unit for the introduction of capacity inseries is really of the form per unit length. Thus if the cable is to beloaded at n points per kilometre, the condensers must have capacities ntimes the value of C as given by the last equation above. Similarly, ifC1, B1 circuits are inserted at 112 points per kilometre, the condensersand resistances must have magnitude mCi and g m:

respectively. If in the original loading unit Cs, R3 was not included,then R3 must be omitted from the equation above, giving C.

If there is any leakance at low frequencies in parallel with thecapacity of the cable, this can be allowed for by modifying the value ofC and shunting it with a suitable leakance G as already described. Itwill be noted that when the added series capacity becomes infinite, i.e., no added capacity is required. It should also be noted that an addedcondenser shunted, by a resistance will correct for a cable where theleakance loss exceeds the series resistance loss. The characteristicimpedance in this case, however, is inductive at the low frequencies.

It must be borne in mind that high frequency correction can be appliedwithout low frequency correction and that low frequency correction canbe applied Without high frequency correction. The magnitude of the lowfrequency correction however depends on whether any high frequencycorrection has been introduced, since high frequency correction altersthe low frequency resistance of the cable.

In cables for operating up to very high frequencies (several megacyclesper second for example) there may be a certain amount of conductance ordielectric loss, which increases the attenuation of the cable "at'highfrequencies. The dielectric loss may be allowed for by considering it asan increase of effective series resistance, producing the sameattenuation, and

designing the capacity-resistance loading circuits accordingly. Anincrease of effective conductance is accompanied by a slight alterationof capacity, the two effects producing a change of attenuation andvelocity analogous to that produced by the change of resistance due toskin effect. The introduction of suitable loading of the type used toneutralise resistance inductance change is also effective inneutralising the effect of capacity conductance change.

The distances between the loadings depend on the frequency rangecorrected by the elements considered. The capacity-resistance circuitswith a long time constant can be separated at wider distan'ces along thecable than capacity-resistance circuits with a short time constant, andsimilarly the C condenser can be put at yet longer spacings. This typeof loading leads to no cut-off frequency,-since the value of the loadingimpedance falls with increase of frequency instead of rising as withseries inductive loading. In general, the loadings should be so spacedthat for a given type of loading the added impedance of the loading issmall compared with the characteristic impedance of the circuit atfrequencies where the distance between loadings represents a quarterwavelength. For example, the impedance of the loading should certainlynot exceed say 10% of the characteristic impedance of the cable for afrequency at which a quarter wavelength occupies the distance betweentwo such loadings,v

The optimum loading distances depend upon the smoothness of anattenuation-frequency characteristic required, and can be determined bytrial and error. For this purpose it is possible to calculate anequivalent T network representing the unloaded cable between two loads.To each of the two cross branches of the T the impedance of half a loadis added. The propagation constant of the T with the additional halfloadingscan then be calculated, and this compared with the propagationconstant deduced for a smoothly distributed loading. Such calculationscan be repeated at a number of frequencies and will indicate thedeparture of the attenuation from that predicted for smooth loading. Anyirregularities in attenuation and velocity will be apparent and if theseare not within the required limits the distances between loadings mustbe reduced.

It will be found convenient to make the loading distances for the lowerfrequency corrections a multiple of the loading distances for thehighest frequency correction, since this will mean the minimum number ofbreaks in the cable. Incidentally,'if a single capacity-resistancecircuit does not adequately correct the cable in a given part of thefrequency range, two slightly dissimilar circuits may be used and put inat alternate loading points. Other similar modifications are possible,

The extra capacity of the condensers and resistances to' earth at theloading point may be corrected for by adding small series inductances oneach side of them, or in the case of a balanced circuit the added mutualcapacity of the loadings of the two wires may be corrected similarly.The value of the total inductance added will be given by 0Z where c isthe added capacity and Z the impedance of the cable. The added straycapacity however, must be kept so small that the added capacity withcorrecting inductances forms a filter with a cut-off well above theworking frequency;

For aerial cables, or cables subject to large variations of temperature,the attenuation-frequency characteristic of the cable will alter withtemperature unless the. loading components are altered suitably with thetemperature of the cable.

Assuming the cable conductors to have a temperature coeflicient 0:, thenadded resistances R1, R2, etc., are also given a temperature coefficientof oz, and if the loading capacities are given a temperature coefiicientof 2oc, the shape of the equalised attenuation frequency characteristicwill remain equalised, but the absolute attenuation will have atemperature coeflicient equal to a. The required temperature coefficientof the resistances may be obtained by choosing a suitable metal. Thetemperature coefiicient of the condensers may be obtained by using asuitable dielectric such as one of the alcohols, or by arranging aloosely stacked condenser which is compressed owing to a reduction oftemperature by some contraction control.

It may be found convenient to make the inner conductor of a concentriccable in the form of a thin metal tube, so as to reduce as far aspossible its change of resistance and inductance with frequency. Thiswill reduce the amount of loading correction required to obtain auniform response in the middle and high frequencies, but will increasethe amount of low frequency correction required in the same manner asadding capacityresistance high frequency correctors increases the lowfrequency correction required.

Series capacitative loading may be intoduced for example either bylumped condensers or by forming the cable conductor or conductors oftwisted wires thinly insulated from one another (e. g., by an oxidefilm) which wires are broken at intervals (the breaks in various wiresbeing out of step) so that the current flows from wire to wire throughthe thin insulation.

Fig. 4 shows a circuit alternative to that of Fig, 3 whereby correctionmay be applied for the high frequency effect. The cable constants are asin Fig. 3 but, instead of the series circuits R1, C1, R2, C2 and R3, C3there are provided shunt circuits each comprising an inductance inseries with a resistance. The resistances are denoted by A1, A2, A3 andthe inductances b-y B1, B2, B3. The variations of the cable constantsmay then be greatly reduced by arranging that L LL L A1=6Ei7 B1=fii 7AZ'ZEJ 8 etc. These expressions for the values of A1, B1, A2, B2, etc.,are obtained from binomial expansions which can only be made on certainassumptions which may not always be justified. The values must thereforebe taken as approximate and correct practical values may be found byexperiment. 1 I

The result of connecting resistance-inductance circuits A1, B1, A2, B2and A3, B3 in-shunt across the line is to produce a cable with constantwave velocity and attenuation at the higher frequencies but, at lowfrequencies, the series inductance re-' mains L+L1+Lz+Ls. The shuntcapacity is however decreased by the effective negative capacityintroduced by A1, B1 etc. The shunt leakance G is increased by r 1 1 1atria As in the case of resistance-capacity loading, the number ofdifferent loading circuits usedfor resistance-inductance loading dependson the frequency range and the degree of approximation required. Asbefore, it may be desirable to correct the cable by loading only up to afrequency short of the highest frequency to be handled and to providecorrection at the highest frequencies in the amplifiers which arenecessary at intervals along the line.

Assuming that A1 and B1 are at the same temperature as the cable, it ispreferably arranged that A1 has a temperature coefficient equal inmagnitude but opposite in sign to the temperature coefficient of theseries resistance of 'the cable, and that B1 has atemperatureboefficient of magnitude double that of the series resistanceof the cable and of opposite sign.

Resistance-inductance loading may also be employed for correcting forthe low frequency effeet. If the low frequency constants are R, C, L, Gas shown in Fig. 5 (these constants must be the effective values andmust therefore allow for the effect of any high frequency correctionwhich has been applied and may therefore be different from thecorresponding values in Fig. 3 or 4) the low frequency correction may beapplied by con necting a resistance A0 and inductance B0 in series withone another and in shunt with the line. The values of A0 and B0 aregiven by L B 9 B E 2 The propagation constant of a cable treated in thisway is then equal to for all frequencies. The first term shows the wavevelocity to be independent of frequency and the other terms show thatthe attenuation is also independent of frequency.

Either the high or the low frequency correction may be appliedseparately to modify the characteristics of the cable over a part or thewhole of the frequency bands over which these corrections are effective.The two corrections may also be applied simultaneously,

The high frequency correction by resistanceincluctance loading may beapplied in conjunction with low frequency correction as described withreference to Fig. 2.

The low frequency correction by resistanceinductance loading may beapplied in conjunction with high frequency correction as described withreference to Fig. 3.

High-frequency correction can also be effected in part byresistance-inductance loading and in part by resistance-condenserloading. If this correction is effected half by the one arrangement andhalf by the other method it is found that a closer approximation tocontinuous loading is obtained and it is therefore possible to space theloading points further apart from one another than when all thecorrection is applied by one only of these arrangements. By suitablydistributing the correction between the two arrangements it may bepossible to arrange that, at low frequencies, GL=RC in which case thecable is uniform for all frequencies and no low frequency correction isnecessary. 7

Similarly the low frequency correction may be applied when necessary inpart by resistance-inductance loading and in part by resistancecapacityloading, whether or not high frequency correction by resistance-capacityor resistanceinductance loading has been applied.

The theory of cables is worked out for continuous loading but inpractice lumped loading is generally applied at loading points suitablyspaced along the cable. For the method of loading byresistance-inductance elements it is found that if a frequency w isinvolved such that A=w B, A being a loading resistance and B theinductance associated therewith, then the distance between consecutiveloading points must be quite small compared with thewavelengthcorresponding to frequency w. The actual distance employeddepends on the accuracy of correction of thecable which is required. Themaximum distance which will satisfy any given requirements is best foundby trial and error, The cable constants are measured and the cable istransformed to its equivalent 1r network. This network is thenterminated at each end by a load equal to one half the load of thecable. The new propagation constant is then determined. It is found thata hump or hollow in the transmission curve of the cable is most likelyto occur at a frequency corresponding to a wavelength one-quarter or onehalf of which is equal to the loading distance. If the curve is'substantially flat in the neighborhood of both these frequencies, thenit is likely to be flat over the frequency range for which the cable hasbeen corrected.

It is to be understood that the calculated values of A and B areexpressed inohms and henries for a unit length. Since these loadingelements are connected in shunt with the cable the units should be thereciprocals of these quantities. It follows that, if the loadingdistance be halved, the values of theloading elements (A and B, etc.)must be doubled.

Referring now to Fig. 1 the cable comprising conductors I, 2 hasconstants denoted by-L, C, R, L1, R1, L2, R2 and L3, Re. The highfrequency effect is corrected by including resistance-capacity circuitsC1, R1, C2, R2 and C3, R: as in Fig, 3 and the low frequency effect iscorrected by series capacity C. As in Figs. 2, 3 and 4 the componentswithin the dotted rectangle 3 represent the constants of the cableitself.

It is to be understood that the cable constants are distributed alongthe line'and thatthe loading impedance elements must, in practice, alsobe distributed along the line. Dotted lines X Y denote the ends of thecable, the feeding and output coupling units being shown outside theselines.

The cable is fed from a valve 4 (shown as a pentode) provided with ananode resistance 5.

-' A coup-ling condenser 6 is connected between the anode l of valve 4and one end of conductor l. The corresponding end of conductor 2 iseartlied. The cable loaded in the manner shown appears like a resistance8 in series with a condenser 9. Since no such components actually existthe resistance 8 and condenser 9 are shown dotted.

It will be seen that at frequencies at which resistance 5 in parallelwith the output impedance of valve 4 is large compared with theimpedance of condensers 6 and 9 in series with one another, constantcurrent will be fed to the cable. By this is meant that when a constantvoltage is applied to the grid 16 at different frequencies, the inputcurrent to the cable is constant, At very low frequencies the impedanceof condensers 6 and 9 becomes high and a constant Voltage feed to thecable results (with constant voltage on the grid l6).

The termination shown in Fig. 1 comprises two condensers II and I2which, in series with one another, have a capacity equal to the capacity9. Resistance I3 is made equal to resistance 3, i. e., the highfrequency surge impedance of the cable. At frequencies at which thevalve 4 is feeding constant current into thev cable, it is arranged thatthe impedance of condenser I2 is small compared with resistance [3 sothat, since the cable is loaded to have uniform attenuation, a constantcurrent output is obtained from the cable and this produces a constantvoltage on the grid !4 ofvalve l5. At very low frequencies the valve .4is feeding a constant voltage to the cable and it is arranged that atthese frequencies the capacity of condenser I2 is such that itsimpedance is large compared with resistance l3 and a constant voltage istherefore applied to grid M of valve 15. By suitably proportioningcondensers H and I2 and resistance [3 with reference to the constants ofthe cable and the elements of the feeding unit, it is possible to obtaina substantially fiat overall response for the equalised range of thecable from the grid 16 of valved to the grid M of valve l5. If the cablehas leakance, the condenser Cf must be'shunted by a resistance asdescribed with reference to Fig. 2 and shunt resistances must also beconnected across condensers H and 12, there being in effect an imaginaryshunt resistance across condenser 9, I

Fig. 6 shows an alternative termination'whioh may be used in place ofthat shown in Fig. 1. Condensers H and I2 and resistance l3 are providedas before but an autotransformer I1 is connected as shown. When theimpedance of the lower portion of auto-transformer H is comparable withthe impedance of condenser l2, an effective voltage step-up is obtainedbetween the cable and the grid M of valve l5. The inductance of thelower'partof auto-transformer I7 is suchthat this step-up realisedonly'over the upper part of the frequency band being handled and valvenoise is thereby kept low. 7 r

A condenser I8 is advantageously shunted across resistance l3, itscapacity being made equal to A 2 where Z is equal to resistance 13(which is also equal to the surge impedance of the cable) and L is theinductance of the lower part of the auto-transformer J1. The-arrangementof Fig. 6 is of particular use in cases where the cable is not equalisedup to the highest frequency which it is desired to handle. I

If desired an arrangement similar-to that of Fig. 6,. can be substitutedfor the feeding unit shown in Fig. 1. The auto-transformer at thefeedingend would preferably have its upper end connected to the righthand terminal of condenser. 6,'its intermediate tapping point connectedto conductor! of the cable and its lower endconnected through aresistance and condenser corresponding to [3, I2 of Fig. 6 to conductor2 of the cable. In this way the autotransformer gives a voltagestep-down, at high frequencies, from the feeding unit to the cable.

Where the cable is of theso-called concentric type (one conductor,insulated from and surrounding the other conductor) the loading ispreferably .all applied to the inner conductor. Theinvention may also beapplied to other cables such, for example, as one comprising a paperinsulated pair of conductors in which series inductance loading is notprovided. The introduction of series condensers into such a cable inorder to correct for the low frequency effect makes the attenuation overthe low frequency range more uniform and it also raises the velocity sothat such pairs form very high quality circuits for audio frequencies.

We claim:

1. An electrical signal transmission system comprising a transmissionline and loading means, said line simulating a smoothline over aworkingfrequency range extending to limiting frequencies at which theattenuation and wave velocity depart from uniformity due tothe finitevalues of the series resistance and shunt leakance of said line, saidloading means comprising resistance and reactance elements, saidelements being equivalent electrically to a condenser in series withsaid line and shunted by a resistance and having magnitudes such as tocompensate for said finite resistance and leakance values, therebypreventing the attenuation and wave velocity to depart substantiallyfrom uniformity in the neighbourhood of said limiting frequencies. v

2. An electric signal transmission system-oomprising a transmission lineand loading means, said line simulating a smooth line over a workingfrequency range extending to limiting frequencies at which theattenuation and wave velocity depart from uniformity due to the finitevalues of the series resistance and shunt leakance of said line, saidloading means comprising resistance and reactance elements, saidelements being equivalent electrically to a condenser in series withsaid line and shunted by a resistance and having magnitudes such thatthe characteristic impedance (Z) of the loaded line is given by theexpression C, L, R and G being respectively the capacity, inductance,resistance and leakance of said line per unit length, thereby renderingthe attenuation and wave velocity of the line substantially uniform inthe neighbourhood of the lower limiting frequency of said range.

3. An electric signal transmission system comprising a transmission lineand loading means, said line simulating a smooth line over a workingfrequency range extending to limiting frequencies at which theattenuation and Wave velocity depart from uniformity due to the finitevalues of the series resistance and shunt leakance of said line, saidloading means comprising resistance and reactance elements, saidelements being equivalent electrically to a condenser in series withsaidlline and shunted by a resistance and having magnitudes such that,in the neighbourhood of the upper limiting frequency of said range, theseries inductance and resistance of said line, together with the loadingmeans, simulate a substantially pure constant resistance and asubstantially constant inductance in series, thereby rendering theattenuation and wave velocity of said line substantially uniform in theneighbourhood of said upper limiting fre quency.

4. An electric signal transmission system comprising a transmission lineand loading means, said line simulating a smooth line over apredetermined working range of frequency, said loading means comprisinga number, including one, of series circuits, each said circuitcomprising a condenser shunted by a resistance connected in series insaid transmission line, said circuits when their number exceeds onebeing spaced from one another at intervals along said line so that themagnitudes (C') and of the effective series capacity and. resistance,

respectively, in a unit length of said line due to said loading meanssatisfy the relationships ere-as 5 C G f 2 Z E 6 wherein C, L, R and Gare respectively the capacity, inductance, resistance and leakance ofsaid "transmission line per unit length.

5. ,An electric signal transmission system comprising a transmissionline and loading means, said line simulating a smooth line over apredetermined working range of frequency, said loading means comprisinga number, including one, of shunt circuits, each said circuit comprisingan inductance in series with a resistance connected in shunt across saidtransmission line, said shunt circuits when their number exceeds onebeing spaced from one another at intervals along said line so that themagnitudes (A) and (B) of the effective shunt resistance and inductance,respectively, in a unit length of said line due to said loading meanssatisfy the relawherein C, L, R. and G are respectively the ca pacity,inductance, resistance and leakance of said transmission line per unitlength.

6. An electric signal transmission system comprising a transmission lineand loading means, said line simulating a smooth line over apredetermined working range of frequency, extending to limitingfrequencies at which the attenuation and wave velocity depart fromuniformity due to the finite values of the series resistance and shuntleakance of said line, said loading means comprising a number, includingone, of series circuits, each said circuit comprising a condensershunted by ,a resistance connected in series in said transmission line,said series circuits when their number exceeds one being spaced from oneanother at intervals along said line so that the magnitudes of theeffective series capacity and resistance due to said loading means aresuch that the rise of resistance and fall of inductance of said linewith increasing frequency are reduced, thereby preventing the atand and

- tenuation and wave velocity to depart substantially from uniformity inthe neighbourhood of said limiting frequencies.

7. An electric signal transmission system comprising a transmission lineand loading means, said line simulating a smooth line over apredetermined working range of frequency extending to limitingfrequencies at which the attenuation and wave velocity depart fromuniformity due to the finite values of the series resistance and shuntleakance of said line, said loading means comprising a number, includingone, of shunt circuits, each said circuit comprising an inductance inseries with a resistance connected in shunt across said transmissionline, said shunt circuits when their number exceeds one being spacedfrom one another at intervals along said stantially from uniformity inthe neighbourhood of said limiting frequencies.

8. An electric signal transmission system comprising a transmission lineand two types of loading means, said line simulating a smooth line overa predetermined working range of frequency extending to limitingfrequencies at which the attenuation and wave velocity depart fromuniformity due to the finite values of the series resistance and shuntleakance of said line, said first type loading means comprising anumber, including one, of series circuits, each said circuit comprisinga condenser shunted by a resistance connected in series in saidtransmission line, said series circuits when their number exceeds onebeing spaced from one another at intervals along said line, said secondtype loading means comprising a number, including one, of shuntcircuits, each said shunt circuit comprising an inductance in serieswith a resistance connected in shunt across said transmission line, saidshunt circuits when their number exceeds one being spaced from oneanother at intervals along said line, each said shunt circuitcorresponding to one of said series circuits of which it is the inversewith respect to the characteristic impedance of the line.

ALAN DOWER BLUMLEIN. JOHN HARDVVICK.

CERTIFICATE CORRECTION. Patent No. 2-,16L ,L 92 Jul L 19 9.

ALAN DOWER BLUMLEIN, ET AL. It is hereby certified tha t the name of theassignee in the above numbered patent was erroneouslydescribedandspecified as "Electrical 8c Musical Industries Limited" whereassaidfname should have been described and specified as Electric & MusicalIndustries Limited, of Hayes, Middleseix,- England, a company of GreatBritain, as shownby the record of assignments in this office; vand thatthe said Letters Patent should be read with this correction therein thatthe same may conform tothe record of the case in the Patent Office.-

Signed and sealed this 29th day of August, A. .D. 1959 Leslie Frazer,(Seal) Acting Commissioner of Patents.

